Semi-Rigid Transmission Line Alternatives
Transmission of signals is becoming evermore complicated and critical in designs and equipment today. The advent of wireless communication; high frequency one- and two-way communication; the need for amplifiers, IF mixers, delay lines, and such; and low power signals have all put higher demands upon transmission line characteristics. In most high frequency microwave applications the preferred solution to this challenge is semi-rigid coax. At the same time, the preferred solution is the most difficult to work with. As a result, several alternative designs are available to the microwave engineer, each with it's own benefits. However, not all these alternatives can be used as direct replacements to semi-rigid coax.
Why replace semi-rigid? Well, it is very expensive to use semi-rigid assemblies. Semi-rigid utilizes a seamless copper tube for the outer conductor, similar to copper water pipe. If you have ever worked with copper pipe, you know that it is difficult to work with. You need special tools and forming devices to bend and shape it. Also, once bent, it is nearly impossible to straighten out or rebend the pipe without causing damage. All these problems can be associated with semi-rigid as well. This translates to dollars. Detailed engineering drawings, special tooling, and extreme care are required with each assembly. This does, however, produce the best available transmission line for its intended application.
There are several other alternatives on the market. Aluminum tube is basically the same as semi-rigid except that the outer conductor is aluminum rather than copper. This presents some advantages since it is hand-formable and can be reformed. Unfortunately, as the bend radii decrease, the material becomes more difficult to work with. As a result, wrinkles and shearing can appear and special tooling (and detailed drawings) may again be required for certain assemblies. Reforming can present physical or electrical problems. Soldering of connectors can be difficult. Also, since aluminum has lower conductivity than copper, attenuation and other electrical properties tend to be degraded.
Flexible cable alternatives are also available. These products typically utilize a silver tape and silver-plated braid or double silver-plated braid with an overall fluoropolymer jacket. Some even tout "low loss" and other enticing verbiage. These products are often difficult to terminate because of their loose tape/braid configurations. Also, the material and construction of these cables inherently lend themselves to cable memory. Cable memory increases the difficulty to route, maintain, or form the cable in a specific shape or location as is typically done with semi-rigid products. Flexible cables will not retain their shape once bent. This may require the use of wire ties or other cable fastening techniques when routing. Finally, all of the silver and additional fluoropolymer jacket material tends to remove the cost advantages associated with replacing semi-rigid cable.
A better alternative is hand-formable products utilizing a tin-dipped braid as the outer conductor. This offers many advantages over the semi-rigid products. The cable is hand-formable. It can be bent and reformed without the use of tools or special assembly drawings. Belden has developed a patented design which rivals the performance of semi-rigid cables. In fact, the semi-rigid military specifications (M17/130E, M17/133D, and M17/151) were utilized as performance guidelines for the new designs. The result is hand-formable, flexible cable which can be used in place of semi-rigid.
Known as ConformableŽ Coax, four standard products are currently available. 1671A is Belden's alternative to RG-405 (M17/133D). It is a 50 ohm cable, .087" in diameter. 1673A is the RG-402 (M17/130E) type, a 50 ohm cable .138" in diameter coax. A 75 ohm .087" diameter version is available for video applications, 1672A. And the newest member of the product family, a 50 ohm 0.047" 1674A (M17/151-type). The electrical information presented here is representative of the RG-402 and RG-405 type cables.
These cables are the same as any other from the core down from a materials standpoint. It consists of a silver plated copper covered steel conductor. The dielectric material is Teflon7 PTFE. Therefore, the advantages in the core come from process techniques and consistency.
The outer conductor consists of a copper foil and an alloy braid material. When tin-dipped, this is called a composite shield. This is the innovative part of the design which Belden has patented.
Why copper tape? The copper tape is applied over the core, foil side out. When the cable is subjected to the tin bath, the tape is effectively soldered together at the tape overlap. This creates a "seamless copper tube" surrounding the core. (Sound familiar?) A phenomena known as skin effect states that as frequency increases, the electronic signal tends to travel on the surface of a conductor. Therefore, as can be calculated from the formula below, this 0.0007" thin layer of copper on the outer surface of the dielectric core is more than sufficient to contain the signals and allow for a highly conductive path at frequencies of interest. This design replicates the semi-rigid coax, as far as the transmission signal is concerned: silver plated conductor - PTFE - copper conductor. It is this outer conductor design which allows Conformable Coax to obtain semi-rigid performance, as can be seen in the attenuation and VSWR charts below.
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The braid consists of an uniquely applied high strength alloy material. The high strength allows the design to enjoy a higher flex life and aids in connector retention. Therefore, bending, flexing, and reforming all become realities in this design solution. Tinned copper, as is typically found in similar designs, is a much softer material which tends to fatigue and shear with flexing and bending.
Because the foil and braid are solder-bonded together, termination of the cable is the same as semi-rigid, whether done by hand or in an automated process. Belden products are available on reels in various lengths for use with automated cutting and stripping equipment as opposed to semi-rigid cables which are available only in 5 ft. to 20 ft. lengths or other products sold in coils.
The composite shield construction provides excellent shield effectiveness. Testing performed on Conformable Coax resulted in readings of RF leakage greater than -140 dB from 0.5 to 2 GHz, -130 dB to 6 GHz, -120 dB to 10 GHz, and -110 dB to 20 GHz. These values were the same for virgin, one, and two flex cycle cable assemblies. After 20 flex cycles, the 20 GHz value decreases only 10 dB, to approximately -100 dB!
Slight differences in diameter can be found between the semi-rigid products and their corresponding Belden alternative. Primarily these are a result of maintaining electrical characteristics of impedance and capacitance. The main issue is connectorization. Belden products can be used with standard solder-on or crimp type SMA-type connectors designed for semi-rigid cables.
Another difference is the temperature rating. While semi-rigid products are rated -40EC to +125EC, Conformable Coax can be used up to +200EC. This is because the unique outer conductor design allows for contraction and expansion of the cable without cracking.
Contamination testing, while not required for semi-rigid cables, has been performed on Conformable Coax as well. The results of salt fog and water submersion testing did not prove detrimental to the cables. This is due in part to the "seamless copper tube" which completely surrounds and seals the dielectric core. Similar hand-formable cables were subjected to the same tests with the opposite results. In tin-dipped braid designs without the copper tape, water migration through the tinned copper braid and along the dielectric core severely degraded attenuation and impedance.
Conformable Coax is ideal for prototype and R&D situations. A great advantage is to have several different length pre-made assemblies available and at your disposal. Simply shape or route the cable by hand and connect the assembly, eliminating the lead-time associated with pre-formed semi-rigid assemblies. Custom versions have been produced for unique applications, such as a variety of tin/lead composite shield for use in European telecommunications satellites currently in orbit! Jacketed versions are also available where isolation of the cable is required.
Belden Conformable Coax is a cost effective alternative to semi-rigid cable. It's use removes the costs associated with drawings, tooling, and special assembly designs. It can be hand-formed again and again to be reused or rerouted. It holds its shape and retains no memory unlike flexible alternatives. It offers excellent VSWR, flex life, connector retention, shield effectiveness, and ease of termination in a hand-formable product.
For further technical product information, contact Belden at 1-800-BELDEN-1 and request new product bulletin NP-106.
ConformableŽ Coax is a registered trademark of Belden, Inc.
Teflon7 is a registered trademark of E. I. DuPont
Written by Marty Van Der Burgt.
Mr. Van Der Burgt is a Senior Product Development Engineer with Belden, Richmond, IN.
